1. Field of the Invention
The invention concerns a magnetic resonance tomography device of the type having a magnet system that is fashioned to generate a gradient field; with a local coil that is fashioned to receive a magnetic resonance signal; and with a localization system that is fashioned to localize the local coil. Moreover, the invention concerns a method to localize a local coil in such a magnetic resonance tomography device, as well as a local coil suitably fashioned for this purpose.
2. Description of the Prior Art
Magnetic resonance tomography (also called MRT) is a technique in widespread use for the acquisition of images of the inside of the body of a living examination subject. In order to obtain an image with this method, i.e. to generate a magnetic resonance exposure of an examination subject, the body or the body part of the patient that is to be examined must be exposed to an optimally homogeneous, static basic magnetic field, which is generated by a basic field magnet of the magnet system of the magnetic resonance tomography device. Rapidly switched gradient fields for spatial coding that are generated by gradient coils of the magnet system are overlaid on this basic magnetic field during the acquisition of the magnetic resonance images. Moreover, RF pulses of a defined field strength are radiated into the examination volume (in which the examination subject is located) with a radio-frequency antenna of the magnetic resonance tomography device. For this purpose, the magnetic resonance tomography device typically has a permanently installed radio-frequency antenna (known as a whole-body coil). The nuclear spins of the atoms in the examination subject are excited by means of these RF pulses such that they are deflected out of their equilibrium state (which runs parallel to the basic magnetic field) by what is known as an “excitation flip angle”. The nuclear spins then process around the direction of the basic magnetic field. The magnetic resonance signals that are thereby generated are generally detected by a non-stationary local coil and made accessible for a further processing. The local coil is arranged optimally close to the patient, for example it is placed on the surface of the patient or test subject, and typically has one or more MRT antennas or antenna coils. For additional processing of the detected magnetic resonance signals it is useful to know the position of the local coil as precisely as possible.
Since the MRT device has no a priori knowledge about the orientation or position of the non-stationary local coil on the bed or on the patient, this must initially be “searched for”. The position of the local coil in the coordinate system of the MRT device is typically sought or determined by magnetic resonance experiments. The elements of the MRT device that are used form a localization system to localize the local coil. The patient bed with the patient located thereon and the non-stationary local coils arranged on the patient are brought into different z-positions along the longitudinal direction of the measurement space within a measurement space (bore) of the MRT device (thus within the magnet system) and a magnetic resonance data acquisition is conducted at the respective z-positions. The orientation or position of the non-stationary local coils can be automatically calculated within certain precision limits from the overview MRT image that is obtained in this manner if a characteristic antenna profile of the MRT antenna contained in the local coil is known.
In this method for localization of the local coil, however, ambiguities that can be caused by (for example) MRT antennas located very close to one another within a local coil pose difficulties. A localization that is implemented in this manner is additionally problematical if the local coil is located outside or at the edge of the homogeneity volume.
To solve this problem, methods could also be used in which the position of the local coil does not first need to be sought with the use of an overview image MRT image, for example. In such methods, MRT-visible markers integrated into the local coil could be used, but these cause additional problems. Namely, if 1H nuclei are used in such a marker, such a marker will then also be visible in the in the MRT image in the subsequent patient examination and will cause artifacts such as aliasing in the phase coding directions. If a marker with a different nucleus (for example a 19F nucleus) is alternatively used, the entire MRT device must be operated at the resonant frequency of this nucleus, which can only be done with additional cost even for nuclei that are very close to the 1H nucleus in terms of resonant frequency (for example the aforementioned 19F nucleus). Again, different methods would have to be adapted with regard to their MRT compatibility, and in many cases no synergies would be present between autarchic systems and the MRT device.